1. Field of the Invention
The present invention relates to an air-fuel ratio control device of an internal combustion engine.
2. Description of the Related Art
A known internal combustion engine comprises an electric purge control valve for controlling the supply of purge gas fed into the intake passage of an engine from a charcoal canister, and an electric air bleed control valve for controlling the amount of air fed into the fuel passage of a carburetor. An electric current fed into the air bleed control valve is controlled on the basis of the output signal of an oxygen concentration detecting sensor (hereinafter referred to as an O.sub.2 sensor) arranged in the exhaust passage of the engine so that the amount of air fed into the fuel passage of the carburetor is increased as the amount of electric current fed into the air bleed control valve is increased (Japanese Unexamined Patent Publication No. 61-1857). In this engine, when the purge control valve is opened, and thus the supply of the purge gas is started, if the purge gas contains a large fuel component, an air-fuel mixture fed into the engine cylinders becomes extremely rich. As a result, the amount of electric current fed into the air bleed control valve is increased so that an air-fuel ratio approaches the stoichiometric air-fuel ratio, and accordingly, the amount of air fed into the fuel passage of the carburetor is increased.
However, there is a limitation to the possible range of air-fuel ratio which can be controlled by changing the amount of air fed into the fuel passage of the carburetor, and thus a problem arises in that, when the air-fuel mixture becomes extremely rich due to the supply of purge gas, even if the amount of electric current fed into the air bleed control valve is increased to the maximum level of the controllable range, the air-fuel mixture is still in a rich state. To solve this problem, in this engine, when the amount of electric current fed into the air bleed control valve is increased to the maximum level of the controllable range, an air-fuel ratio control is changed from the air-fuel ratio control based on the air bleed control to the air-fuel ratio control based on the purge control, and thus the amount of purge gas is controlled so that an air-fuel ratio approaches the stoichiometric air-fuel ratio.
Note, fuel vapor produced, for example, in the fuel tank, is fed into the charcoal chanister, and the fuel component of the fuel vapor is adsorbed in the activated carbon of the canister. In this case, as time elapses after the adsorption, the fuel component penetrates deeper into the activated carbon and is firmly retained therein. However, there is a limitation to the amount of fuel component which can be adsorbed in the activated carbon, and thus if the fuel component is retained in the activated carbon, the amount of fuel component which can be newly adsorbed in the activated carbon is reduced by the amount of fuel component already retained in the activated carbon. That is, if the activated carbon with the fuel vapor adsorbed therein is not disturbed for a long time, the adsorbing ability of the activated carbon is gradually reduced. Consequently, to prevent a reduction of the adsorbing ability of the activated carbon, as much as possible of the fuel component adsorbed in the activated carbon must be desorped, so that the fuel component is not retained deep in the activated carbon.
However, where the amount of purge gas is controlled as in the above-mentioned engine, since the amount of purge gas fed into the intake passage of the engine is reduced, the amount of fuel component which is retained in the activated carbon is increased, and as a result, since the amount of fuel component penetrating deep into the activated carbon and retained therein is increased, a problem arises in that the adsorbing ability of the activated carbon is reduced.